Recently, core biopsy devices have been combined with imaging technology to better target a lesion in breast tissues. One such commercially available product is marketed under the trademark name MAMMOTOME™, by Ethicon Endo-Surgery, Inc. An embodiment of such a device is described in U.S. Pat. No. 5,526,822 issued to Burbank, et al., on Jun. 18, 1996, and is hereby incorporated herein by reference. Its handle receives mechanical and electrical power as well as vacuum assist from a remotely positioned control module that is spaced away from the high magnetic field of a Magnetic Resonance Imaging (MRI) machine.
As seen from that reference, the instrument is a type of image-guided, percutaneous coring, breast biopsy instrument. It is vacuum-assisted, and some of the steps for retrieving the tissue samples have been automated. The physician uses this device to capture “actively” (using the vacuum) the tissue prior to severing it from the body. This allows the sampling of tissues of varying hardness. In addition, a side opening aperture is used, avoiding having to thrust into a lesion, which may tend to push the mass away, causing a track metastasis, or causing a hematoma that, with residual contrast agent circulating therein, may mimic enhancement in a suspicious lesion. The side aperture may be rotated about a longitudinal axis of the probe, thereby allowing multiple tissue samples without having to otherwise reposition the probe. These features allow for substantial sampling of large lesions and complete removal of small ones.
In MRI, the presence of both the magnetic and RF fields used in the imaging process place several constraints on each instrument to be positioned or manipulated near or in the imaging region of the MRI system. The MRI system imposes a strong constant magnetic field (e.g., 1 Tesla) to align electrons of the atoms of the body. Then a magnetic gradient is applied to disturb these aligned electrons. As the electrons return to alignment, a weak RF signal is emitted that must be detected and interpreted. Compatibility with such an environment requires that the instrument must be essentially non-ferromagnetic, so that it is not attracted by the magnetic field, which would pose a safety problem. This consideration applies to any object that is used near or that is inserted into or implanted within the patient being imaged, because the magnetic field subjects such an object or implants, if ferro-magnetic, to undesirable forces and torques. In addition, an electrical instrument should be tolerant of the static and pulsed magnetic and RF fields in order to be operable in the presence of these fields. Further, an implant or instrument should not be unduly subjected to induced heating due to eddy current from the applied RF field. Finally, the instrument should not create excessive imaging artifacts that obscure or distort the image of the target.
To address these constraints, MRI compatible biopsy instruments are generally assembled from non-ferrous materials; however, other materials that are MRI imageable are sometimes used. In some instances, imagability relies upon the lack of an MRI RF return image to contrast with the image returned by adjacent tissue. Also, ferromagnetic particles or liquid lumens for holding aqueous paramagnetic ions are sometimes incorporated.
While these generally-known MRI biopsy devices provide MRI compatibility and a degree of imagability, further improvements would be desirable. More particularly, a significant need exists for an MRI compatible biopsy device that enhances locating a sampling aperture in an MRI compatible penetrating portion, even in an MRI scan slice that obliquely passes through the probe. Positive identification of the sampling aperture location in the presence of obscuring factors such as contrast infused body fluids, gas introduced by the procedure has significant value to the clinician.